Nowadays, foam is widely used as a waterproof sealing material in various fields such as civil engineering, construction, electrical engineering, electronics engineering and vehicles. Examples of foam to be used as a waterproof sealing material include thermoplastic resin foam made of polyethylene-based resin and polypropylene-based resin or the like, and rubber foam made of synthetic rubber or natural rubber.
Among these sealing materials, the waterproof sealing materials are used for filling up interspace existing in various structures such as architectural structures, electrical equipment and vehicles in order to prevent water from entering those structures. Such waterproof sealing materials are arranged in a portion to be sealed in a compressed state to generate a repulsive force which restores the waterproof sealing materials to original shape. Then, the repulsive force causes the waterproof sealing materials to adhere tightly to the interface of the portion to be sealed without any interspace.
Here arises a problem that when the compression flexibility of the waterproof sealing material is low, the repulsive stress of the waterproof sealing material becomes so high as to deform the portion to be sealed. In addition, there is another problem that the deformation of the portion to be sealed may cause enlargement of the interspace of the structure and decrease of an adhesion property of the waterproof sealing material to the portion to be sealed (hereinafter to be referred to as “sealing capability”). The decreased adhesion property of the waterproof sealing material results in its poorer waterproofing capability.
In view of these problems, open-cell foam superior in compression flexibility has been used as the above-mentioned waterproof sealing material. Such open-cell foam, however, suffers from a problem that the cells are communicated with each other to thus easily permit the passage of water into the foam, resulting in poorer waterproofing capability. Also when an open-cell foam absorbs water and swells in contact with water to improve sealing capability, such open-cell foam is inferior in sealing capability until a sufficient water absorption, and may thus permit the passage of water before reaching the full sealing capability.
Also used as the above waterproof sealing material is another type of foam product having both closed cells and open cells, the former contributing to improved waterproofing capability and the latter contributing to improved compression flexibility. For example, Patent Document 1 discloses a fixed-form sealing material in which a foamed structure having both closed cells and open cells is provided with a cell skin having a water-absorptive, swelling property, and the number of cells per cm in length is eight or more.
In the above fixed-form sealing material, however, there arises a problem that its repulsive force decreases during a long-term use, causing its sealing capability to be lowered down to an insufficient level of waterproofing capability.
Therefore, Patent Document 2 discloses a rubber-based resin closed-cell foam sheet made of a foam structure having closed cells with a superior interfacial contact with a portion to be sealed, and also a highly functional waterproof sealing material using such closed-cell foam sheet. Nevertheless, even higher waterproofing capability has been desired.